Is diplomacy necessary?

Diplomacy is a useful thing; you can even avoid entering a castle. Just gather an army along the way, as long as you have leadership, otherwise, they'll make fun of you, or at least they'll complain.

Gathering creatures from different castles (the first ones you encounter) and attacking the enemy with such a composition without even entering a castle to reinforce your army is simply tactless.
However, it's a different story if you gather such an army at the beginning of the game and go to capture mines or resources guarded by mediocre monsters. But in this case, the hero is unlikely to have the diplomacy skill that allows monsters to join him (especially if their strength is only slightly less than yours...).